1. Field of the Invention
The present invention relates to novel radiation-curing powder coating binders containing maleic and/or fumaric acid esters and to the use of these binders as a constituent of coating compositions.
2. Description of the Prior Art
The use of radiation-curing pulverulent binders offers the inherent advantage over heat-curing powders that flow and curing are separately controllable. It is therefore conventional firstly to melt the applied powder coating and allow sufficient time to enable the desired flow, and then to cure the coating by means of energy-rich actinic radiation.
To be able to coat temperature-sensitive substrates as well, the melting step should be carried out at the lowest possible temperatures (≦120° C.) for the shortest possible time in order to assure a good flow of the coating composition and so as not to damage the substrate.
Radiation-curing binders are based on the chemical systems known in powder coating chemistry, such as polyesters carrying acrylic groups or polyacrylates. Urethane acrylates are used for particularly high-quality coatings.
Whereas EP-A 410 242 describes the preparation of such binders in organic solvents, DE 199 398 43 teaches the solventless preparation thereof using a high proportion of 10-95 wt. % of a crystalline component synthesized from a monoisocyanate or diisocyanate and a monohydroxyalkyl (meth)acrylate.
Although very good coating properties have been obtained, the storage stability of these powder coatings is poor. The powder cakes in the grinding and sieving process, so industrial use is only possible at cooled ambient temperatures that are not practical.
WO-A 03 044 111 describes storage-resistant, pulverulent amorphous urethane acrylates which can be ground at room temperature, have no tendency to cake and have a particularly low viscosity after melting. The glass transition temperatures of these compounds are above 45° C. To obtain a good flow of the powder coating binders described, it is necessary to use melting temperatures above 120° C., which are too high for some important substrates such as certain plastics and types of wood.
The object of the invention was therefore to improve the known radiation-curing powder coating systems so that satisfactory flow properties are achieved even at melting temperatures of ≦120° C. without grindability and storage stability being adversely affected.